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What you are describing is unspecified and implementation-defined behavior [0].

Avoiding UB (edit: in general) doesn't have anything to do with the code being portable and everything with the code not being buggy [1][2].

[0] https://en.cppreference.com/w/c/language/behavior

[1] https://blog.regehr.org/archives/213

[2] http://blog.llvm.org/2011/05/what-every-c-programmer-should-...

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Oh really? Then why does every compiler I use have a parameter to turn off strict aliasing?

You cite to a source that contradicts you. In the llvm blog post: "It is also worth pointing out that both Clang and GCC nail down a few behaviors that the C standard leaves undefined."

Sometimes a compiler gives a guarantee that a particular UB is always handled in a specific way, but you cannot generalize this to all UB.

Added 'in general' to my comment to make this explicit.

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